Fluorescent lamps are well known and comprise a tubular hermetically sealed glass envelope including electrodes at ends thereof. Inside the envelope is an arc discharge sustaining medium, usually at a low pressure, including inert gases and a small amount of mercury. The inside of the glass envelope is typically coated with a layer of phosphor, which absorbs ultraviolet electromagnetic radiation of 254 nm and 185 nm generated by the excited mercury arc and emits in a region of visible light.
Fluorescent lamps usually experience a gradual decrease in light output (measured in lumens) with the increase of lamp usage (measured in hours burned). Ideally, the phosphor should absorb the 254 nm and 185 nm emission strongly and convert them into visible light efficiently. But in reality, most of the 185 nm wavelength radiation is wasted, which lowers the overall efficiency of the lamp. Moreover, 185 nm emission also leads to formation of color center—a type of point defect—in phosphors, which decreases the phosphor conversion efficiency and lumen output of lamps over their life cycle. There are other notable problems associated with phosphors in fluorescent lamps. The phosphor coating is exposed to both ion bombardment and chemical reaction from the mercury discharge which is a reducing medium. In addition, during phosphor synthesis and lamp fabrication process, phosphors are usually exposed to oxygen-rich atmosphere which tend to partially oxidize reactive lower-valence ions in the phosphor lattice. These problems lead to the overall degradation of phosphors and their lumen output over life.
Yttrium vanadate phosphors (e.g., YVO4: Eu3+, Y(P,V)O4: Eu3+) strongly emitting in the deep red region, have been widely utilized in high pressure mercury lamps, and have greatly improved color rendering properties of these lamps. However, they have been excluded from current commercial use in fluorescent lamp manufacturing because of their very severe lumen depreciation during lamp operation.